Revert "chore: disable warnings in old CI (#4691)"

This reverts commit b24df49a9d so we can
merge the CHANGELOG changes in #4691 into master.

tests-build/tests/fail/macros_dead_code.rs

@@ -0,0 +1,8 @@
+#![deny(dead_code)]
+
+use tests_build::tokio;
+
+#[tokio::main]
+async fn f() {}
+
+fn main() {}